1. Field of the Invention
The present invention refers in general to percussion tooling that can be used in perforation, demolition or excavation operations.
2. Background Art
Known percussion tools of the type mentioned above comprise a first portion, that, when operating, is housed at least partly inside the hammer, and a second portion that includes a working bit. Normally, such tools are manufactured in a single piece of a material, typically steel, chosen in order to satisfy a compromise between two opposite needs. In fact, the first portion of these tools, whose upper end is cyclically subjected to impacts of a high amount due to the percussion action performed by a beating mass moving inside the hammer, requires a mean hardness and a high flexural strength and a high resistance to impacts, and therefore a good elastic resistance. The second portion of such tools, that is subjected to impacts against the material to be demolished, requires a high hardness and toughness and a high resistance to hot wear. Since such known tools are made in a single piece, steel used for making them is chosen in order to be able to simultaneously satisfy the required requirements for both above portions, and therefore has not wholly optimum characteristics either for the first or for the second portion. In particular, for the whole tool, steels are used that are alloyed with elements such as nickel, molybdenum, chromium, vanadium and similar metals, many of which, being rather costly, affect the global tool cost.
It would instead be desirable to be able to have tools available of the above-defined type whose material has different characteristics for each portion, so that the two portions are suitable for supporting the types of specific stresses to which each one of them is subjected during use.
In principle, it can be deemed that the first portion of the tool provides a contribution approximately for 85% of the weight, while the second portion, equipped with the working end, gives a contribution approximately for 15% of the weight. The two tool portions could be made separately with different materials, each one chosen to optimally bear its related stresses. In this way, the first portion could be made of a chromium-molybdenum or chromium-manganese alloyed steel, relatively inexpensive, while the second portion could be made of a highly specialized steel, with high resistance to hot wear, for example alloyed with tungsten and cobalt, and therefore relatively more expensive but only in relation to a small part of the tool.
In this way, much better tool performances could be obtained with greater reliability and use length, at a lower cost than the one required for making the tool wholly with a high-performance steel alloy.
There is anyway the problem, having a difficult solution, of managing to guarantee a connection of these two portions that is rigid, stable in time and reliable. In particular, the most common types of connection, for example of the screw and nut screw type, are absolutely unsuitable to resist in time to the high pulse stresses to which a tool of the type herein described is subjected.
Document BE-A-440648 discloses a percussion tool according to the prior art.